Various wireless communications techniques may utilize beamforming to transmit signals via multiple transmitter antennas, thereby increasing the throughput of the system (see, for example, IEEE 802.11n, IEEE 802.11ac, IEEE 802.16e, IEEE 802.16m, LTE Advanced, etc.). For example, a wireless communications device (such as a wireless access point) may transmit a single data stream via multiple transmitter antennas. As another example, a wireless communications device may transmit multiple distinct data streams concurrently to multiple receiver antennas via multiple transmitter antennas—an approach referred to as Multiple-Input-Multiple-Output (MIMO). A communications device that utilizes beamforming to transmit wireless signals may be referred to as a beamformer.
Beamforming involves varying parameters (such as the phase or amplitude) of the individual signals transmitted by the multiple antennas, so as to manipulate the combined radiation pattern that is emitted by the antennas in some desired way. Certain MIMO techniques may utilize a particular form of beamforming known as precoding, which is characterized by having multiple distinct data streams that are transmitted with independent weightings (as opposed to transmitting the same data stream from each antenna). The parameter variations (e.g., the weightings) that are applied to the signals may be represented in a matrix, which may be referred to as a steering matrix. The beamformer and/or the client device may compute a steering matrix that will produce some desired result (e.g., optimize throughput), and the beamformer may apply the steering matrix to the signals that are to be transmitted.
MIMO techniques include Single-User MIMO (SU-MIMO) and Multi-User MIMO (MU-MIMO). In SU-MIMO techniques, multiple data streams may be transmitted concurrently on the same frequency channel to a single client device via multiple transmitter antennas, the combined signal may be received at multiple receiver antennas of the client device, and the client device may differentiate the data streams from the combined by exploiting knowledge of the wireless channel. In MU-MIMO techniques, multiple data streams may be transmitted concurrently on the same frequency channel to multiple client devices via multiple transmitter antennas, the combined signal may be received at each of the client devices, and the client devices may each recover their intended data stream from the combined signal by exploiting knowledge of the wireless channel.